My school does not offer any accelerated courses. Whatsoever. There is just Science, Math, English, History, Latin, and an elective. I know it is important to challenge yourself in high school, so what should I do as a substitute?
Get amazing grades in those classes and find something you are passionate in and be amazing at it. Like sports or writing(a book).
@andrewl2k Thanks! But are there any substitutes for lack of a rigorous course load, such as taking college courses at a nearby university or self studying?
Yeah, you just said them 
Well, there are several programs that offer online AP classes. Just google them. You could also possibly take some classes at your local community college.
Try to self study some AP classes and take the exams. Like southernbelle16 said, take some higher level online classes.
Also, because you lack the opportunity to take AP courses (which are supposed to be college level) a very important option to consider is taking classes at your local community college! These are great because it shows that you are challenging yourself and are self-motivated, and can handle college level work. Also, many colleges will accept these credits which can possibly allow you to skip some requirements/gen eds. If you don’t have AP courses available, I think that classes at your CC are the next best thing. If you need help paying for them, try to talk to your guidance counselor or talk to other kids that have done the same thing. I know at my school, everyone who has a certain GPA can take one CC class for free each semester. Also, if you are in Educational Talent Search (ETS), or a similar program, they will sometimes pay for your classes and/or textbooks. If they don’t pay for your textbooks (which can be expensive), ask around to see if anyone you know has one you can borrow, or look online on eBay or some other website to find cheap used ones. Additionally, self-studying AP classes is a good option too. It allows you to get credit for those (depending on school policy). Keep in mind though, many of the top schools won’t give college credit for scores less than a 4, and for foreign language exams usually a 5. Although, if you are aiming for your local state college, many will take 3s. Ok good luck!